The present invention relates to semi-hard magnetic materials in which both magnetized state functioning as a magnet and demagnetized state having substantially no magnetism can be used, bias materials for magnetic markers, magnetic markers which use the bias materials for magnetic marker, and production methods of bias materials for magnetic markers.
A semi-hard magnetic material is a material having a middle coercive force between a so-called permanent magnet also called hard magnetic material and a soft magnetic material. The semi-hard magnetic material shows soft magnetism in the matrix having magnetism by itself but by dispersing a nonmagnetic material therein so that the dispersed nonmagnetic material functions as obstacles against domain wall displacement when a magnetic field is applied from the outside, the coercive force thereof can be adjusted by the form and amount of the dispersed materials.
The semi-hard magnetic material is a material which can be used by changing between the two states of the magnetized state and the demagnetized state as stated above. Therefore, it has been used for switching devices for changing the direction of electric current, actuators and shutters which drive and/or control flow, sensing parts, etc.
In late years, electronic monitoring systems comprising attaching a magnetic label and detecting the label as a marker have been suggested for the purpose of theft prevention, recognition of article flow and kind of article, and so on. These systems are based on a mechanism consisting of resonantly oscillating a magnetostrictive element made of a certain amorphous metal in an alternating magnetic field, thereby changing the magnetic field and detecting the change by a pick-up coil to activate alarm.
An element which applies a bias magnetic field to a magnetostrictive element is called a bias element. Markers are attached to articles in the store in the state in which a semi-hard magnetic material is magnetized. When a price for an article is paid, an attenuating magnetic field, which decreases in the absolute value while repeatedly reversing the direction thereof, is applied from the outside of the marker, whereby the magnetic force of the bias element is reduced to equal to or less than the magnetic force in the fully magnetized state and the resonance of the magnetostrictive element is decreased, which inactivates the marker so that the alarm may not be activated.
When a semi-hard magnetic material is used for a magnetic marker, for example, the coercive force thereof must not be too small since the state of the bias element must not be changed by a magnetic field inadvertently applied from the outside. In the meantime, it is important to have an appropriate coercive force since inactivation becomes difficult if the coercive force is too large. Therefore, as materials of semi-hard magnetic materials which can have such an appropriate coercive force, the present inventors have proposed Fe—Cu group semi-hard magnetic materials (JP-A-11-12698).
The Fe—Cu group semi-hard magnetic materials of JP-A-11-12698 contain a large amount of Cu in Fe, and Cu, which is a nonmagnetic material, is finely deposited to increase the number of obstacles against the domain wall displacement thereby increasing the coercive force. JP-A-11-12698 also discloses a technique to disperse carbides such as those of Nb in addition to fine precipitation of Cu. It was supposed in JP-A-11-12698 that, when used as bias elements for magnetic markers, around 10 to 20% by mass % of Cu was necessary to obtain a coercive force of around 1600 to 2000 A/m which was considered to be suitable in bias elements.
However, there was a large difference between the solidification temperatures of Fe and Cu in the Fe—Cu alloy of JP-A-11-12698 and therefore, it was difficult to obtain a uniform structure by melt preparation method in which the alloy was molten and poured in a mold to be solidified since solidification of Cu delayed in the ingot casting. That is, macroscopically, Cu is segregated in the core of the ingot and microscopically, Cu aggregate in the grain boundary with Fe which constitutes a matrix.
Accordingly, the present inventors have proposed hot isostatic press method using powders (hereinbelow referred to as powder HIP method) as a realistic approach to finely disperse a large amount of Cu in Fe and obtain a uniform structure, which was difficult by melt preparation method (JP-A-11-12698). Here, the powder HIP method is a method to obtain uniform materials by using alloy powders obtained by a quenching method and filling the powders in a can, and compact pressurizing the same.
This powder HIP method is a method to finely disperse Cu in Fe and thereby obtaining a uniform structure by compound effect of Cu and various additional elements such as Mo and W by adding the various elements in a powder state.
Besides, the present inventors have suggested a laminating rolling method (JP-A-2000-150219) as another approach to let disperse Cu finely dispersed in Fe. The laminating rolling method disclosed in JP-A-2000-150219 is a method comprising alternately laminating a layer A mainly composed of Fe and a layer B mainly composed of Cu group non-magnetic metal to obtain a laminate, subjecting the laminate to heating and plastic working thereby particularly dividing the layer B having a low melting temperature to obtain a uniform material.
It is undeniable that the HIP method and the laminating rolling method disclosed in JP-A-11-12698 and JP-A-2000-150219 are high-cost production methods as compared with ordinary melt preparation method since the powders and plates or foils used as raw materials in themselves are costly and a number of steps are needed to obtain materials from the raw materials. In order to produce Fe—Cu group semi-hard magnetic materials industrially and inexpensively, industrial scale-up is effective and melt preparation method is the most excellent, and it is desirable to perform production by producing ingots as large as possible.
However, since the melting point of Cu is lower than that of Fe, aggregate region of Cu becomes more fluid (or liquefied) particularly at the time of hot processing in the melt preparation method, and cracks tend to occur from the aggregate region of Cu leading to deterioration of hot workability. Particularly, materials containing Cu in a content of 10.0% or more by mass % have a problem that they are remarkably poor in hot workability and difficult to be used in melt preparation method although the method is suitable for mass production. Therefore, it was preferable to reduce the Cu content as much as possible, but there was caused a problem that coercive force decreases when the Cu content is reduced.
An object of the present invention is to solve the problem in hot workability caused by Cu and provide semi-hard magnetic materials having high coercive force with less Cu content than conventional materials, bias materials for magnetic markers, magnetic markers and production methods of bias materials for magnetic markers.